1. Field of the Invention
The invention relates to luminal endoprostheses formed principally of a framework, without textile covering, generally called “stents”, and more particularly to stents for blood vessels, and i.a. for blood vessels bearing junctions.
2. Description of the Related Art
Over the years, the implantation of luminal endoprostheses has become an approved technique for treatment of aneurysms, atherosclerosis, etc.
However, one crucial problem has still not been solved: namely that of perfectly matching the mechanical and hemodynamic characteristics of these endoprostheses and of the arteries in which they are implanted.
Even if very particular care has been taken to meet these criteria at the time of implantation, a disparity invariably develops in the long term. This is because the human body is subject to changes due to aging, while the endoprosthesis has a problem of stability over the course of time: tearing of the filaments, deterioration of the structure, possible increase in diameter (by loosening of the structure) and inadequate interaction with the flow.
The mechanical characteristics of a stent are determined essentially by the structure of its framework. Although different types of these exist, such as the frameworks made up of flat braids described in WO 99/55256, the most suitable framework at present is the cylindrical braided framework, such as is described in particular by Didcott in GB-1205743, or in U.S. Pat. No. 5,061,275.
This type of framework compresses easily for insertion, resists well to crushing and retains a relative flexibility compatible with that of the blood vessels; the structure adapts to the sinuous course of the rigid arteries to be treated.
To date, investigations into finding the optimum framework have focused on the choice of material, the braiding pitch, etc. These investigations inevitably come up against a number of practical problems. By adopting a very small braiding pitch (the angle between the axis and the spirals being close to 90° or by choosing thick wires, the radial force (resistance to crushing) is increased, which means a high rate of shortening. Conversely, a large pitch, where the angle formed between the axis and the spirals is close to 30° for example, and the use of thin wires give the framework good flexibility but a low resistance to crushing and thus a low radial force to resist to artery compression. This problem is even more critical for stents and endoprostheses made up of several modules cut by laser.
Attempts have been made, particularly by Thompson (U.S. Pat. No. 5,718,159), to combine metal wires with textile fibres. However, the results obtained are not convincing: the metal filaments deform the structure and, along their helical course, they create dislocations and spacing of the textile fibres. The fibres are subjected to stresses under the effect of the pulsations caused by the blood flow and they are subject to rapid erosion-fatigue by friction against the metal filaments (whose modulus of elasticity and diameter are greater). Furthermore, Thompson suggests to use for his endoprosthesis a framework made out of mere metal (“structural”) filaments, covered by one or two impervious layers. This suggestion is purely theoretical. Indeed, tests prove that it would be impossible to obtain a permanently stable structure using metal filaments braided after they have been subjected to a thermical hardening, as suggested. Such an endoprosthesis would be brittle or at least unstable, the pre-stiffened wire being unable to bend plastically at their crossing points. Furthermore, a property described as basic in U.S. Pat. No. 5,718,159 is the fact that the endoprosthesis should be impervious.
Results based on recent clinical studies have shown that, in the case of an aneurysm of the abdominal aorta, 70% of the pressure wave is transmitted to the wall of the aneurysm via the endoprosthesis. (Reference: Communication at the 27th Global Vascular Endovascular Issues Techniques Horizons™ Nov. 16-19, 2000, page V5.1). These findings are not surprising because haemodynamics teach us that when the walls are thin, the necessary work implied for the transport of the blood increases. It is also known that when the vessels are too large, the volume of blood increases beyond what is necessary. These factors promote aneurysms.
In the case of the Superficial Femoral Artery and of the popliteal arteries, one has also to take into consideration the fact that they move in multiple plans during the limb motion in 3D directions. These arteries are thus not only compressed and rotated, but they are also shortened and extended in response to the movement. All these factors are known to negatively impact the long-term use of stents.
This shows that more stable and more robust structures have to be developed. Investigations based initially on the above consideration revealed that a far more important field of possibilities had been neglected by the searchers, namely the influence of the stents on the haemodynamic in blood vessels in general cases as well as in the particular case of aneurysms.